1. Technical Domain
The invention concerns a process for cleaning the ink nozzle or nozzles of an ink jet printer.
The invention also concerns a cleaning device using this process.
The invention also relates to a print head with one or more nozzles incorporating such a cleaning device, as well as printers comprising at least one such print head.
The invention can be used in all ink jet printers, whether of the continuous ink jet or xe2x80x9cdrop on demandxe2x80x9d type.
2. State of Technology
As illustrated, in particular by document U.S. Pat. No. 3,373,437, in a continuous ink jet printer, a print head delivers at least one ink jet through a calibrating orifice supplied with ink under pressure. This ink supply comes from an ink reservoir that is either connected to a pump or pressurised by means of gas. Each jet is then broken down into droplets of ink, which are electrically charged by charging electrodes, in such a way that they are either deflected or not deflected by electrodes situated downstream. Depending on whether or not they are deflected, the droplets either will or will not be printed on a substrate situated downstream. At least one solenoid valve, situated within the supply line connecting the reservoir to the print head generally allows the flow of ink to be stopped when the printer is not running.
Printers that operate according to this technique may use inks incorporating volatile, very quick-drying solvents, or resins for ensuring good adhesion to difficult substrates, or even pigments in dispersion allowing opaque markings to be applied to dark substrates.
In xe2x80x9cdrop-on-demandxe2x80x9d type printers, the ink droplets are released intermittently by a nozzle located in the wall of an ink chamber maintained at a less than atmospheric pressure. The chamber is supplied with ink from a reservoir under the simple effect of capillary forces. A piezo-electric or thermal transducer causes the droplets to be ejected by deforming the wall of the chamber.
In each of these two techniques;, the reliability of operation depends mainly on the conditions at the orifices, i.e., the state of the nozzles through which the ink is ejected.
These conditions are particularly difficult in xe2x80x9cdrop on demandxe2x80x9d type printers, as the intermittent nature of their operation means that ink can remain standing in the nozzle for long periods of time. The inks used in printers of this type are thus very slow drying. Moreover, a large number of devices exist that are intended to avoid the ink drying on the nozzles and to guarantee that the consistency of the ink remains perfectly constant in the vicinity of the ejection orifice to ensure the proper ejection of droplets.
In continuous ink jet printers, it is easier to maintain the area immediately around the ink nozzle in clean condition when the jet is operating, as the bulk of the ink is then in movement and the risk of the ink drying is lower than in xe2x80x9cdroplet on demandxe2x80x9d type printers.
On the other hand, with continuous ink jet printers, there is a very brief phase during the start-up of the jet that is particularly delicate. This is when the printer changes from a state where the ink is at rest in the reservoir to one in which a continuous high speed ink jet is established. Indeed, during this phase, the slightest obstruction to the flow of ink in the nozzle can significantly deflect its trajectory. This deflection may cause ink to come into contact with sensitive printer components situated downstream of the nozzle, such as the charging or deflecting electrodes, which are live.
The characteristics of the jet establishment phase in a continuous ink jet printer are very similar to those of the intermittent ejection of ink in a xe2x80x9cdroplet on demandxe2x80x9d type printer. It is for this reason that the solutions initially developed for one of these two technologies are generally transferred to the other.
One of the most difficult problems to resolve in ink jet printers relates to the drying of the ink in the vicinity of the outside face of the nozzle when the jet is stopped. These residues may be caused by ink splashing during printing or simply by a projecting contact point of the meniscus formed by the ink inside the nozzle during operation or when the jet is stopped. This phenomenon is particularly critical in certain industrial applications using continuous ink jet printers, which use quick-drying, highly adhesive ink.
Many solutions have already been proposed for avoiding deflected jets at the start-up of continuous ink jet printers and/or to limit the consequences. However, none of these solutions gives entire satisfaction.
A solution that is known to limit the consequences of jet deflection at start-up consists in using retractable electrodes, that are placed out of reach of any jets that may be deflected during the start-up phases. This solution is relatively effective but is onerous to implement if the operator is required to manually move the electrodes. It is also expensive, due to the level of precision required for the alignment of the mobile electrodes.
The majority of known solutions seek rather to ensure start-up without deflected jets. These solutions can also be combined with those above.
A first known solution for avoiding deflection of jets start-up consists in cleaning the outer face of the nozzle by hand before each start-up, for example using a washing bottle, with or without mechanical brushing. This type of cleaning frequently requires subsequent drying of the surface of the nozzle using an air jet. Depending on the type of ink used, the damp residues may also be removed by mechanical scouring. This solution is particularly effective, but it is lengthy and not very ergonomic for the user, and its success is very dependant on the skill of the operator.
Another known solution for avoiding deflection of jets at start-up is described in document WO-A-91/00808. When the jet stops, a vacuum is created in the upstream chamber in order to avoid the expulsion of unwanted droplets of fluid in the vicinity of the ink meniscus as it is stabilising. The system is completed by a device for obstructing the orifice of the nozzle, situated on its upstream face. This solution avoids the ink from drying in the chamber and guarantees that the inside of the nozzle is clean, as the in the chamber is hermetically isolated from the outside air. This system does not guarantee the cleanliness of the outside face of the nozzle, however, which may have been wet by ink splashes during the start-up of the jet or during the printing phase.
Another known solution for avoiding deflection of jets at start-up is described in document U.S. Pat. No. 5,706,039. This solution consists in rinsing the nozzle from channels incorporated in the outer face of the nozzle plate.
This solution does not guarantee efficient or complete cleaning of the outside face of the nozzle, however, when the ink residues are highly adhesive. Moreover, it does not allow air drying. A certain amount of solvent therefore risks to remain around the nozzle, thus contributing to the deviation of the jet.
A fourth known solution for avoiding deflection of jets at start-up consists in totally immersing the print head housing in a solvent. This radical solution, which is described in document WO-A-99/01288, presents the problem of drying the elements of the print head that have been immersed. It also does not perform a mechanical action on the external face of the nozzle when this is required. Moreover, this solution leads to a high level of cleaning solvent consumption, which is neither cost effective or environmentally desirable on account of the large amount of liquid waste produced.
Document GB-A-2 316 364 describes an alternative version of the previous solution, in which a chamber of limited volume is attached to the charging electrode and placed in contact with the outer face of the nozzle. The chamber can be in turn filled with cleaning solvent or emptied of solvent residue by suction. This solution significantly reduces the volumes of liquid used. It does, however, have the same shortcomings of the previous solution regarding the absence of mechanical action and drying.
A further known solution for avoiding deflection of jets at the start-up of continuous ink jet printers is described in document WO-A-86/06026. In this case, an external, retractable nozzle cleaning accessory is mounted on the outer face of the nozzle. This solution is costly and difficult to implement, due to the additional apparatus it requires. Moreover, cleaning consists simply of immersing the nozzle, which is frequently insufficient when highly adhesive ink is used. Solvent consumption and the volume of waste also remain high.
As described in particular in document EP-A-0 437 361, another known solution consists in wiping and scraping the outer surface of the nozzle using thin, flexible blade suited to this purpose. However, the choice of material for the scraping blade is difficult for printers using solvent inks. Moreover, this solution requires a cumbersome device for controlling the relative movement of the nozzle and the scraper.
All of the previous solutions can be used with nozzle plates whose surfaces have been treated to reduce their wettability and minimise ink adhesion, as described in document FR-A-2 747 960.
A final known solution consists of systematically sealing the end face of the nozzle when the jet stops, by means of a contact valve as explained in document EP-A-0 017 669. The effectiveness of this solution is uncertain when using quick drying inks, however, and it does not guarantee that the cleanliness of the external face of the nozzle when the valve opens.
In conclusion, none of the known solutions to date can perform all of the essential operations necessary for ensuring the proper operation and total reliability of the print head after the jet has stopped, in a simple and inexpensive manner, regardless of the type of ink used.
The specific object of the invention is a nozzle cleaning process performing all of the operations necessary for the proper operation and total reliability of the print head in a simple and inexpensive manner, using no moving or retractable elements, using a small volume of solvent, generating small amounts of waste, in a manner adapted to the characteristics of the ink, as required, in other words, spaying the external face of the nozzle with solvent, while simultaneously performing local mechanical action, scraping off residues and removing them from the area around the nozzle, and perfectly drying and removing all traces of solvent after cleaning.
According to the invention, this result is achieved by a process for cleaning at least one ink nozzle of an ink jet printer after the jet has stopped, said process being characterised by the fact that it comprises the following successive stages:
the spraying of cleaning solvent towards the ink nozzle, at an angle to the ink jet, from a fixed cleaning jet situated downstream of the nozzle.
the blowing of dry air towards the front face of the ink nozzle from said cleaning jet.
In the process thus defined, the solvent leaving the cleaning jet is sprayed onto the nozzle in a cone of fine droplets ejected at high speed. The micro-droplets hit the area around the nozzle to be cleaned. The mechanical impact of the droplets and the subsequent streaming of the solvent on the front face of the nozzle plate result in effective cleaning. The angle of inclination of the solvent spray relative to the front face of the nozzle allows the ink residue to be scraped off and removed away from the immediate vicinity of the nozzle by friction. The waste ink is projected against the inside face of the print head housing, in an area remote from the electrodes.
The wetting of the nozzle with solvent, the simultaneous local mechanical action, the scraping of residue and its removal well away from the nozzle area are thus ensured simply and inexpensively when the solvent is sprayed by the cleaning jet.
The dry air that is then blown by the cleaning jet also allows the area around the ink nozzle to be dried and the ink residue to be deposited on the inside of the housing.
According to a preferred embodiment of the invention, the orifice of the cleaning jet used has a diameter of between five and fifteen times that of the ink nozzle.
Moreover, the cleaning jet is best positioned downstream of the ink nozzle, and at distance of between five and fifteen times the diameter of the cleaning jet.
The volume and pressure of the solvent and air supplied to the cleaning jet are preferably adjusted to suit the nature of the ink used in the printer.
In the preferred embodiment of the invention, the cleaning jet is supplied with cleaning solvent at a pressure in excess of 100 mbars.
It is best to control the supply of solvent and air to the cleaning jet by means of two solenoid valves or one three-way solenoid valve.
The printer will preferably be provided with a porous surface to recover the residues resulting from cleaning, said surface to be situated downstream of the ink nozzle and opposite the cleaning jet relative to the ink nozzle.
The invention also concerns a device for cleaning at least one ink nozzle of an ink jet when the jet is stopped, said device being characterised by the fact that it comprises a fixed cleaning jet located downstream from the ink nozzle and able to spray cleaning solvent, then blow dry air towards the ink nozzle, at an angle to the ink jet, when the device is operated.
The invention also relates to a print head containing at least one ink nozzle and a device for cleaning same, in the embodiment just defined.
The invention also concerns a printer containing at least one such print head.